Cystic kidney disease (CKD), a leading cause of renal failure, is proposed to originate from an underlying defect in ciliary physiology, though the molecular mechanisms are unclear. The goal of this grant proposal is to elucidate the molecular etiology of CKD in the Thm1aln/aln mouse model, which exhibits a defect in ciliary protein trafficking that leads to increased Hedgehog (Hh) signaling. Interestingly, mutations in THM1 have recently been identified in patients with nephronophthisis, Bardet-Biedl Syndrome and Meckel-Gruber Syndrome, which feature CKD as a major clinical component (N. Katsanis, personal communication). Importantly, the loss of GLI2, the primary transcriptional activator of Hh signaling, rescues most of the kidney phenotype in Thm1aln/aln,Gli2-/- double mutants, implicating overactive Hh signaling in the etiology of Thm1aln/aln renal cysts. In the first aim, the role of excessive GLI2 activity in Thm1aln/aln renal cystogenesis will be investigated by using small molecule Hh antagonists to rescue cystogenesis in cultured mutant kidneys. In the second aim, the molecular mechanism of GLI2 activation and its modulation by THM1 will be explored by using these same small molecule Hh antagonists to examine the relationship between GLI2 ciliary trafficking and activity in Thm1-deficient inner medullary collecting duct (IMCD) cells using live-cell imaging, and by assessing GLI2 protein stability and the status of Suppressor of Fused, a negative regulator of the GLI proteins, in Thm1-mutant extracts. These analyses will elucidate the molecular events connecting a ciliary defect to renal cystogenesis, as well as the interconnection between ciliary physiology and GLI2 activation. This proposal advocates a role for Hh signaling in the etiology of CKD, which has been largely unexplored, and combined with the use of small molecules, can potentially lead to novel therapeutic approaches. PUBLIC HEALTH RELEVANCE: Proven therapies lack for cystic kidney disease (CKD), a leading cause of renal failure, which is proposed to originate from an underlying defect in ciliary physiology, though the molecular mechanisms are unclear. Recently mutations in THM1 have been identified in patients with nephronophthisis, Bardet-Biedl Syndrome and Meckel-Gruber Syndrome, which feature CKD as a major clinical component (N. Katsanis, personal communication). This proposal aims to elucidate the molecular etiology of CKD in the Thm1aln/aln mutant mouse, which exhibits a unique ciliary defect leading to increased Hedgehog (Hh) signaling, by utilizing small molecule Hh inhibitors to rescue cystogenesis in cultured mutant kidneys and to visualize their effects on ciliary physiology of Thm1-deficient, kidney-derived mammalian cells using live-cell imaging.